PerThreadSet: make sure that the pthread setspecific callback does not access destroyed memory

This caused the 'self->map.size() == self->map_elts' assert we saw sometimes on travis CI

src/core/thread_utils.h

@@ -182,6 +182,7 @@ private:
     std::tuple<CtorArgs...> ctor_args;
 #ifndef NDEBUG
     int map_elts = 0;
+    bool got_already_destroyed = false;
 #endif
 
     static void dtor(void* val) {
@@ -190,6 +191,8 @@ private:
 
         auto* self = s->self;
 
+        ASSERT(!self->got_already_destroyed, "");
+
         LOCK_REGION(&self->lock);
 
         assert(s->my_tid == pthread_self());
@@ -237,6 +240,15 @@ public:
         ASSERT(this->map.size() == this->map_elts, "%ld %d", this->map.size(), this->map_elts);
     }
 
+    ~PerThreadSet() {
+        LOCK_REGION(&lock);
+        ASSERT(this->map.size() == this->map_elts, "%ld %d", this->map.size(), this->map_elts);
+        pthread_key_delete(pthread_key);
+#ifndef NDEBUG
+        got_already_destroyed = true;
+#endif
+    }
+
     void forEachValue(std::function<void(T*)> f) {
         LOCK_REGION(&lock);
         ASSERT(this->map.size() == this->map_elts, "%ld %d", this->map.size(), this->map_elts);